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[IAI,LV] Avoid creating a scalar epilogue due to gaps in interleave-groups when

Description

[IAI,LV] Avoid creating a scalar epilogue due to gaps in interleave-groups when
optimizing for size

LV is careful to respect -Os and not to create a scalar epilog in all cases
(runtime tests, trip-counts that require a remainder loop) except for peeling
due to gaps in interleave-groups. This patch fixes that; -Os will now have us
invalidate such interleave-groups and vectorize without an epilog.

The patch also removes a related FIXME comment that is now obsolete, and was
also inaccurate:
"FIXME: return None if loop requiresScalarEpilog(<MaxVF>), or look for a smaller
MaxVF that does not require a scalar epilog."
(requiresScalarEpilog() has nothing to do with VF).

Reviewers: Ayal, hsaito, dcaballe, fhahn

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D53420

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